$(function () {
    $.ajax({
        url: APP_PATH + "/checkLoginStatus.action" ,
        type: "get",
        dataType: "json",
        success: function (data) {
            // alert(U_id);
            // alert(data.user_name);
            if (data.user_name == undefined) {   // 未登录
                alert("未登录");
                window.location.href("/bbs/");
            }
        }
    })
});